1. Field of the Invention
The present invention relates to the structure of a semiconductor device having a high withstand voltage and a high reliability, and more particularly, to a bipolar transistor, and a method of fabricating such a semiconductor device.
2. Description of Related Art
FIG. 5 is a sectional view showing a conventional npn transistor of a Double Polysilicon Self-Alignment (DPSA) structure.
In FIG. 5, reference numeral 1 denotes a semiconductor substrate (silicon substrate); 2 a semiconductor region (epitaxial layer); 3 separating insulation film (field oxide film); 4 an external base electrode; 5 a silicon oxide film; 6 an external base diffusion layer; 7 an intrinsic base diffusion layer; 8 a silicon oxide film; 9 an emitter electrode; 10 an emitter diffusion layer; 11 a silicon oxide film; 12 an external base contact electrode; 13 a collector contact electrode; 14 an emitter contact electrode; and 15 a protection film.
FIGS. 6A-6D are sectional views showing the steps of a method of fabricating a conventional npn transistor of a DPSA structure.
The method will be explained, as shown in FIG. 6A, an external base electrode 4 is first formed on the top surface of an npn transistor element region separated by a separating insulation film (field oxide film) 3 in a semiconductor region (epitaxial layer) 2 on a silicon semiconductor substrate 1, and an external base forming impurity is implanted.
Next, as shown in FIG. 6B, an external base diffusion layer 6 is formed by making an opening in a part of the intrinsic base region on the external base electrode 4 by photolithography, and heat-treating.
Next, as shown in FIG. 6C, an intrinsic base diffusion layer 7 is formed by implanting an intrinsic base forming impurity through the oxide film formed by heat treatment.
Next, as shown in FIG. 6D, after forming side walls 8, an emitter electrode 9 is formed, and an emitter forming impurity is implanted to form an emitter region 10.
Conventional npn transistors of a DPSA structure fabricated as described above have the following problems:
(1) In the region linking the intrinsic base diffusion layer 7 with the external base diffusion layer 6, an electric field is concentrated, a leakage current increases, and withstand voltage lowers.
(2) Since the intrinsic base diffusion layer 7 is formed by implanting an intrinsic base impurity into a delicately opened region, the impurity concentration profile at the edge of the opening has a large curvature, and leakage current increases, lowering withstand voltage.
(3) Since the external base diffusion layer 6 is formed by the diffusion from the external base electrode 4, the linkage at the edge of the separating insulation film (field oxide film) 3 becomes shallow, increasing leakage current and lowering withstand voltage.
The object of the present invention is to solve the above problems, and to provide a semiconductor device having a high withstand voltage, a high reliability, and a stable performance, in particular a bipolar transistor, especially an npn transistor, and a method of fabricating such a semiconductor device by
(1) relaxing the electric field in the region linking the intrinsic base diffusion layer with the external base diffusion layer;
(2) reducing the curvature of impurity concentration profile in the intrinsic base diffusion layer; and
(3) reducing leakage current at the end of the separating insulation film (field oxide film).
According to a first aspect of the present invention, there is provided a semiconductor device comprising an intrinsic base diffusion layer formed on the substantially central portion of a semiconductor region surrounded by a separating insulation film on the major surface of a semiconductor substrate; an external base diffusion layer overlapping with the outer circumference of said intrinsic base diffusion layer, surrounding said intrinsic base diffusion layer, and reaching the separating insulation film; and a common base diffusion layer formed in the semiconductor region, overlapping with said intrinsic base diffusion layer, overlapping with at least the inner circumference of said external base diffusion layer, and formed in a depth deeper than the depth of said external base diffusion layer but not exceeding the depth of said intrinsic base diffusion layer.
In the semiconductor device, the common base diffusion layer may be formed so as to overlap with the intrinsic base diffusion layer and the external base diffusion layer, and to reach the separating insulation film.
In the semiconductor device, the impurity concentration of the external base diffusion layer may be higher than the impurity concentration of the intrinsic base diffusion layer, and the impurity content of the common base diffusion layer may be lower than the impurity content of the intrinsic base diffusion layer.
In the semiconductor, the impurity concentration of the external base diffusion layer may be higher than the impurity concentration of the intrinsic base diffusion layer, and the impurity content of the common base diffusion layer may be lower than the impurity content of the intrinsic base diffusion layer.
According to a second aspect of the present invention, there is provided a method of fabricating a semiconductor device comprising the steps of forming, on the major surface of a semiconductor region of the first conductivity type formed on a semiconductor substrate, an external base diffusion layer of the second conductivity type in the outer circumference region other than the substantially central portion of a semiconductor region surrounded by a separating insulation film; forming an intrinsic base diffusion layer deeper than the external base diffusion layer on the substantially central portion of the semiconductor region surrounded by the separating insulation film; and implanting a common base impurity obliquely and rotationally into the substantially central portion of the semiconductor region, and forming common base diffusion layer of the second conductivity type having a depth deeper than the depth of the external base diffusion layer but not exceeding the depth of the intrinsic base diffusion layer.
According to a third aspect of the present invention, there is provided a method of fabricating a semiconductor device comprising the steps of forming, on the major surface of a semiconductor region of the first conductivity type formed on a semiconductor substrate, a common base diffusion layer of the second conductivity type by obliquely and rotationally implanting a common base impurity into a semiconductor region surrounded by a separating insulation film; forming an external base diffusion layer of the second conductivity type shallower than the common base diffusion layer in the outer circumference region other than the substantially central portion of the common base diffusion layer; and forming an intrinsic base diffusion layer deeper than the common base diffusion layer on the substantially central portion of the common base diffusion layer.
In the method of fabricating a semiconductor device, the impurity concentration of the external base diffusion layer may be made higher than the impurity concentration of the intrinsic base diffusion layer, and the impurity concentration of the common base diffusion layer may be made lower that the impurity concentration of the intrinsic base diffusion layer.
In the method of fabricating a semiconductor device, the impurity concentration of the external base diffusion layer may be made higher than the impurity concentration of the intrinsic base diffusion layer, and the impurity concentration of the common base diffusion layer may be made lower that the impurity concentration of the intrinsic base diffusion layer.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.